1. Field of the Invention
The present invention relates to rolling and, in particular, to methods and mills for rolling metal strip.
2. Description of the Prior Art
There is known a method and an apparatus for rolling metal strips (L. F. Coffin Jr., Rolling method and apparatus, U.S. Pat. No. 3,309,908).
The method consists substantially of the rolling being conducted, in accordance with FIG. 1, in at least three work rolls, "A", "B" and "C", successively contacting one another through the strip "D" being rolled. The middle work roll "C" forms with the extreme work rolls "A" and "B" two deformation zones and its axis is offset with respect to the plane passing through the axes of said extreme work roll "A" and "B". In addition, the middle work roll "C" is idle and free to move under the tensioning action of the strip "D". The speeds of rotation of the work rolls increase along the path of rolling, adjacent rolls rotating in opposite directions.
According to FIG. 1, the strip "D" goes around the extreme work roll "A", deforms in a first deformation zone formed with the work rolls "A" and "C", passes around the work roll "C", deforms in a second deformation zone formed with the work rolls "B" and "C", and then goes around the second extreme work roll "B". The distance between said deformation zones along the circular arc of the work roll "C", which is the path of motion of the strip "D", is always greater than half the length of its circumference.
The above-mentioned patent also describes mills for rolling metal strips comprising a stationary working stand wherein placed three at least work rolls, two at least thereof being secured in chocks arranged in the working stand housing and provided with a drive means for rotating them in opposite directions at different peripheral speeds increasing along the path of rolling. At least one roll is provided with a screwdown. In such mills the axis of the middle work roll is offset with respect to the plane passing through the axes of the extreme work rolls in the direction opposite to the entry of the strip metal being rolled into the first pair of work rolls formed with the middle and the first extreme work rolls, along the path of rolling and to the exit of the strip metal from the second pair of work rolls, formed with the middle and the second extreme work rolls.
The known method and mills for rolling metal strips suffer from a host of disadvantages which can best be illustrated by reference to FIG. 1.
FIG. 1 shows the arrangement of the work rolls and the strip being rolled and the forces acting upon the middle work roll "C" on the side of the first extreme along work roll "A" the path of rolling and the strip "D" being rolled.
According to FIG. 1, the condition of equilibrium of the middle work roll "C" may be written in the form EQU P.sub.z -T.sub.z =0 (1)
where P.sub.z and T.sub.z are the components of the forces "P" and "T" along the Z-axis. Then EQU P.multidot.cos .beta.-T.multidot.sin .beta.=0 (2)
i.e., EQU P=T.multidot.tan .beta. (3)
Hence, the pressure "P" upon the strip being rolled is determined by the value of the tension "T" of said strip "D".
Due to a limited tensile strength of the strip and the risk of its rupturing, the specific tension practically used is never more than half the yield limit of the material involved (0.5.delta.y).
Thus, the condition (3) takes on the form: EQU P.ltoreq.0.5.delta.y.multidot.b.multidot.h.multidot.tan .beta., (4)
where
b=width of the strip; and PA1 h=height of the strip.
Therefore, the possible pressure "P" and the deformation of the strip being rolled are very limited.
Another major disadvantage of the known rolling method and apparatus is that there is no possibility of using small diameter work rolls to reduce the pressure of the metal upon the rolls, in particular, when rolling wider sheets. This is because in the known rolling method, it is impossible to use backup rolls for minimizing the bending of the work rolls, since the strip being rolled goes around said work rolls.
In addition to the above described mills, also known are other types of mills for analogous applications having the same arrangement of work rolls and characteristic of which are the aforesaid disadvantages (J. Franek, Apparatus for Rolling Strip Metal, U.S. Pat. No. 3,253,445; P. H. Hume, Reduction of Elongated Bodies, U.S. Pat. No. 2,332,796, and others).